Iterative Cement Bond Logging Without Calibration

Cement bonding logging (CBL) has recently evolved to include logging-while-drilling (LWD) sonic tools due to the multiple benefits of LWD logging. Yet, there is an LWD-specific challenge, which is the contamination of the casing mode by the drill collar mode. To resolve this issue, existing approaches require calibration in 100% bond zones or free-pipe zones. This calibration step involves human intervention and could pose a hindrance to accurate CBL if these zones cannot be correctly identified. This paper proposes an automated process for iterative CBL using an LWD tool. This process does not require calibration based on the identification of 100% bond zones or free-pipe zones. The proposed iterative CBL starts from preprocessing. The preprocessing workflow results in three variables: the magnitude at the first receiver (RX1), the apparent attenuation (AppAtt), and the apparent attenuation difference (delta AppAtt). After obtaining RX1, AppAtt, and delta AppAtt, we can calculate bond indices in the main processing. The calculation requires the prediction of the summation model, a theoretical model indicating the relation between AppAtt and the real attenuation RealAtt. We pass field data through the workflow in Fig. 1. In Track 4, we show two possible RealAtt values inverted from the right and left branches of the summation model. Track 6 shows the branch indicator, which has an initial random distribution. Based on the branch indicator, we choose one from the two possible RealAtt at each depth and then obtain the RealAtt log. Next, we calculate the correlation coefficient between RX1 and RealAtt. Supposing that the coefficient has reached a minimum after several iterations, we then convert RealAtt into cement bond indices by linear mapping, as shown in Track 2. However, since the branch indicator is initialized randomly and the correlation coefficient does not reach a minimum, the resultant bond indices jump frequently, which is incorrect. To minimize the error in the final bond index log, we need to iteratively update the branch indicator depth by depth until the correlation coefficient between RX1 and RealAtt reaches a minimum. The results are shown in Fig. 1. The final bond index log is very close to the benchmark values in Track 2, validating the proposed workflow. We propose an automated processing workflow for iterative CBL using an LWD tool. This new processing utilizes the trend of amplitude log change instead of individual amplitude values and thus is ideal for processing where we cannot calibrate amplitude logs. Furthermore, unknown parameters, such as the branch indicators and the model parameters, are iteratively updated to achieve reasonable cement bond indices without human intervention. This iterative LWD CBL method enables quantitative cement bond logging without calibration or human intervention and is applicable to post-processing LWD CBL as well as onsite and real-time processing if required.